Ruth Murray-Clay

Precise atmospheric abundances of giant exoplanets are becoming increasingly prevalent, and there is a need to systematically convert these measurements into metrics that inform us about the planets’ accretion history. We present a quantitative framework for relating atmospheric composition to the accretion of metals and primordial gas. This includes (i) new relations that simplify and generalize the connection between atmospheric metallicity and metal mass fraction, (ii) methods for inferring the source of the metals and quantifying the quantity of metals accreted via different sources, and (iii) relating the derived quantities to expectations from formation models. We also derive a new estimate of the minimum disk mass needed to form a planetary system based on the amount of excess metals accreted by a planet, assuming that this excess originates from the drift and evaporation of pebbles at condensation fronts. This Letter is accompanied by an open-source code that implements the methods presented herein.

AF Lep b is one of the closest analogs to Jupiter in terms of mass (3–4 M _Jup ) and semimajor axis (9 au) amenable to spectroscopic characterization. We present JWST/NIRSpec high-contrast spectroscopy of the planet from 2.85 to 5.3 μ m at R ∼ 3000, which provide detections of CO ₂ , H ₂ S, CH ₄ , ¹² CO (and ¹³ CO), and H ₂ O, as well as complementary JWST/NIRCam imaging that captures the planet’s continuum flux from 4.0 to 4.7 μ m. Combining the JWST observations with spectra from VLTI/GRAVITY and VLT/SPHERE (1.0−2.5 μ m), we carry out atmospheric retrievals that include the effects of clouds and disequilibrium chemistry while allowing the C, O, and S abundances to vary independently. AF Lep b exhibits metal enrichment across C, O, and S with C/H = 2.9 ± 0.5, O/H = 3.7 ± 0.6, and S/H = 4.7 ± 0.7 times solar (and stellar). The planet’s slightly subsolar C/O and C/S are consistent with formation near its observed location, and disfavor formation beyond the CO snowline. The sulfur enrichment in AF Lep b implies significant accretion of disk solids during formation, and we estimate the planet contains 56 ± 7 M _⊕ of solids. The C, O, and S enrichment levels of AF Lep b are similar to those of Jupiter and other super-Jupiters like HR 8799 bcde. We also show that the degree of atmospheric metal enrichment of these imaged planets is similar to the bulk metal enrichment of transiting gas giants with masses greater than ∼1 M _Jup , suggesting that the process and efficiency of metal accretion for gas giants may not be strongly dependent on orbital distance or planet mass.

The debris disk orbiting the M star AU Microscopii (AU Mic) has a series of large-scale clumps that move away from the star at high velocities above the midplane on the southeast side. Two more bright features lie on the northwest side of the disk, localized below the midplane and moving toward the star. These clumps are only observed in scattered light, indicating that they affect small 0.2 μ m sized grains. We present a mechanism for emitting periodic dust clumps by appealing to stellar forces and an inclined, eccentric planet interacting with an exterior debris disk. In our best-matching simulations, the planet exerts an impulse on the disk every orbital period, generating periodic enhancements in dust above the midplane. We assume that the stellar wind only acts on grains once they reach a height above the midplane that exceeds a threshold value (a free parameter in our model), at which point they are accelerated outward. The behavior of periodic particle ejections and trajectories depends significantly on the planet’s mass, eccentricity, and inclination; separation between the planet and disk; and the ratio of stellar wind force to the star’s gravitational force ( β ). We find a promising qualitative match to observations with simulations that include an as-yet-undiscovered and observationally allowed planet with mass 2 M _J , semimajor axis between 3 and 4 au, eccentricity of 0.37, and inclination of 30 ^∘ , a ring of particles between 5 and 6 au, and a stellar wind height threshold of z _h = hr , where h ≈ 0.02. We visualize our simulation with surface brightness maps to compare with existing observations of AU Mic. We find that a value of β ≈ 1.8 accelerates the clumps radially outward at velocities that are comparable to the clumps seen in the AU Mic disk and produces features similar to those observed.

Warm Jupiters—giant planets with orbital periods of ∼10–365 days—exhibit a positive correlation between mass and eccentricity. We interpret this trend as the outcome of planet–planet scattering, representing a transition from collision-dominated interactions among low-mass planets to ejection-dominated interactions among high-mass planets. This framework has important implications for the origins of warm Jupiters. It suggests that warm Jupiters originate from compact, multiplanet configurations. The dynamical interactions that shape their present-day architectures likely occur near their current semimajor axes, regardless of whether warm Jupiters formed through convergent disk-driven migration or in situ formation. We argue that several observed properties of warm Jupiter systems, including the eccentricity bimodality, the mass–eccentricity relation, and generally low stellar obliquities, can be explained by this picture. We further predict that not only circular warm Jupiters, but also eccentric warm Jupiters, should frequently have additional planetary companions that are detectable through radial velocity observations. Finally, scattering can produce eccentricities high enough to trigger high-eccentricity tidal migration, potentially explaining the emerging population of proto-hot Jupiters on tidal migration tracks.

With four giant planets ( m ∼ 5 M _Jup −10 M _Jup , T _eff ∼ 900–1200 K) orbiting between 15 and 70 au, HR 8799 provides an unparalleled test bed for studying giant planet formation and probing compositional trends across the protoplanetary disk. We present new JWST/NIRSpec integral field unit observations (2.85–5.3 μ m, R ≈ 2700) that now include the spectrum of HR 8799 b and higher signal-to-noise ratio spectra for HR 8799 c, d, and e compared to those in J.-B. Ruffio et al. We detect CO, CH ₄ , H ₂ O, H ₂ S, CO ₂ , and, for planet b, NH ₃ . We combine the NIRSpec spectra with 1–5 μ m photometry to perform atmospheric retrievals that account for disequilibrium chemistry and clouds, and we allow C/H, O/H, N/H, and S/H to scale independently. While the four planets are similarly enriched in carbon and oxygen, with C/H and O/H between 3× and 5× stellar, we observe a tentative trend of increasing S/H—a tracer of refractory solids—from 2× to 5× stellar with increasing orbital distance. From HR 8799 b’s NH ₃ abundance, we estimate N / H = 21 . 2 − 8.8 + 16.2 × stellar, suggesting that the outer planet accreted significant amounts of N-rich gas. Overall, the elemental abundance patterns we observe are consistent with a picture where planet b formed between the CO snowline and the more distant N ₂ snowline, while the inner planets accreted 3× stellar CO-enriched disk gas within the CO snowline. The excess volatile mass from pebble drift and evaporation implies an integrated pebble flux of 750 ± 200 M _⊕ . The increase in the planets’ S/H with orbital distance implies more solid accretion further out, which is quantitatively compatible with expectations from both pebble and planetesimal accretion (2× minimum-mass solar nebula) paradigms.

In this study, we compute completeness-corrected occurrence rates of giant exoplanets as a function of mass, semimajor axis, and eccentricity, using the approximately uniform California Legacy Survey sample of radial velocity-discovered planets published in L. J. Rosenthal et al. We recover the previously detected rise in occurrence with semimajor axis for both lower- and higher-mass subsets of the population out to ∼5 au. When restricting to planets with semimajor axes between 0.1 and 4.5 au (roughly speaking, the “peak” of giant planet occurrence), we find evidence for distinct eccentricity distributions for each of two mass subpopulations. Most strikingly, we observe a peak in the eccentricity distribution of super-Jovian planets (3–20 M _J ) at 0.3, which is apparent using two different parameterizations of the eccentricity distribution model. A hierarchical histogram model reveals that ∼92% of posterior samples indicate an elevated occurrence rate of super-Jupiters with modest eccentricities (0.2–0.4) compared to lower or higher eccentricities (i.e., evidence for a moderate eccentricity “peak”), and 99% of samples indicate super-Jupiters with modest eccentricities are more common than those with lower eccentricities (i.e., evidence that moderate eccentricities are more common than low eccentricities). We use a truncated Gaussian model fit to pinpoint the location of the super-Jupiter eccentricity peak with more precision, finding a maximum a posteriori peak location of e = 0.3. This low but elevated characteristic eccentricity could be the result of dynamically hot histories, perhaps involving a giant impacts phase. All analysis code for this project is publicly available on Zenodo (doi: 10.5281/zenodo.18089157 ) and GitHub: github.com/sblunt/eccentricities .

We develop a flexible one-dimensional code to model the escape of hydrogen and helium from a hot Jupiter as a result of photoionization from extreme-ultraviolet (EUV) radiation. We include stellar spectrum heating and ionization, radiative cooling by Ly α and H 3 + , heat conduction, tidal gravity, and a H–He reaction network, and account for the secondary ionization of species by photoelectrons. For a fiducial hot Jupiter, we uncover a three-layer structure: an H 3 + -cooled layer of molecular hydrogen at the base, enveloped by a Ly α -cooled layer of neutral hydrogen, which transitions into an ionized wind layer that is cooled by adiabatic expansion. The highest spectral energy photons are deposited in the molecular layer, where, after accounting for energy loss via photoelectrons and ionization, H 3 + is a substantial radiative coolant. We run a grid of models, varying the distance of our fiducial planet from the star. We find that heat conduction at the base starts to have an effect at distances ≳0.2 au, increasing the H 3 + cooling relative to the EUV input flux. At increasing stellocentric distances, the outflow becomes increasingly more neutral. The neutral hydrogen starts to decouple from the ionized outflow, free-streaming out. In pure H–He mini-Neptune/super-Earth simulations, the outflows are significantly cooler, allowing molecules to survive throughout the outflow, pointing toward the likely importance of molecular cooling in determining whether these planets can maintain massive atmospheres. The analysis in this paper provides a framework for understanding the impact of molecular radiative cooling on atmospheric outflows.

Throughout their lives, short-period exoplanets ( M ̇ ), metals decrease M ̇ , and the net result for a typical hot Jupiter is a similar M ̇ but a hotter, faster, and more gradually ionized wind. We find that mulitfrequency photons (e.g., 13.6–2000 eV) are absorbed over a broader range of heights in the atmosphere, resulting in a wind launch radius, R _XUV , that is of order 10 nanobars for all but the highest surface gravity planets. Grids of H/He solar-metallicity atmospheres reveal that, for typical hot Jupiters like HD 209458b, R _XUV ≈ 1.1 R _P –1.8 R _P for low fluxes, meaning that the energy-limited mass-loss rate, M ̇ Elim ( R ) , computed at R = R _P is a good approximation. However, for planets with low escape velocities, like many sub-Neptunes and super-Earths, R _XUV can be ≫ R _P , making it necessary to use M ̇ Elim ( R = R XUV ) to avoid significantly underestimating mass-loss rates. For both high escape velocities and large incident fluxes, radiative cooling is significant and energy-limited mass loss overestimates M ̇ .

We model the optical and infrared transient ZTF SLRN-2020, previously associated with a star-planet merger. We consider the scenario in which orbital decay via tidal dissipation led to the merger, and we find that tidal heating within the star was likely unobservable in the archival image of the system taken 12 yr before the merger. The observed dust formation months before the merger is consistent with a planet of mass M _p ≳ 5 M _J ejecting material as it skims the stellar surface. This interaction gradually intensifies, leading to significant mass ejection on a dynamical timescale (≈hours) as the planet plunges into the stellar interior. Part of the recombination transient associated with this dynamical mass ejection might be inaccessible to the optical observations because its duration (≈hours) is comparable to the cadence. Correspondingly, the observed duration of the transient (≈100 days) is inconsistent with a single episode of dynamical mass ejection. Instead, the transient could be powered by the recombination of 3.4 × 10 ⁻⁵ M _⊙ of hydrogen in an outflow, or the contraction of an inflated envelope of mass ≈10 ⁻⁶ M _⊙ that formed during the merger. The observed ejecta mass 320 days after the peak of the optical transient is ≈1.3 × 10 ⁻⁴ M _⊙ , consistent with the idea that a fraction of the ejecta might be unobservable in the light curve. Energetically, this post-merger ejecta mass suggests a planet at least as massive as Jupiter. Our results suggest that ZTF SLRN-2020 was the result of a merger between a star close to the main sequence and a planet with mass at least several times that of Jupiter.

Super-puffs are a class of low-mass, large-radius planets that have challenged planet formation and evolution models. Their high inferred H/He mass fractions, required to explain their physical sizes, would lead to rapid atmospheric escape, raising questions about their long-term retention. Recent modeling work indicates that low-mass planets typically require 50% less H/He mass to match their observed radius, due to the significant roles of the radiative atmosphere and interior heating from the rock/iron core. Here, through a new quantitative analysis of X-ray and EUV (XUV)–driven escape in sub-Neptunes, we find that previous studies overestimated mass loss, as scaling laws in low-gravity regimes deviate greatly from the widely used energy-limited regime. We define a new regime, thermal-energy-mediated photoevaporation, in which thermal-energy conversion critically sets the mass-loss rate. These effects make super-puffs more resilient to mass loss than previously thought. We develop a coupled evolution model integrating this updated thermal evolution framework with a 1D hydrodynamic photoevaporation model. Applying this novel, joint model to observed super-puffs and young low-density planets, we find that their masses, radii, and transit pressures align with predictions assuming either a clear or hazy atmosphere. This indicates that super-puffs have undergone a combination of boil-off and photoevaporative mass loss, with boil-off dominating the process. Our results indicate that low-density planets typically possess both a thick convective envelope and substantial radiative atmosphere, which contribute to their large radii. For this to occur, these planets must have intermediate masses of 5–10 M _⊕ and receive stellar insolation ≲30 F _⊕ , favoring FG-type stars over M dwarfs.